This invention relates to a process for separating 2,5-dihydrofuran (2,5-DHF) from aqueous solution of 2,5-DHF by extractive distillation using certain organic liquids as the extraction agent.
U.S. Pat. No. 4,231,941 discloses that 2,5-DHF can be synthesized by the simultaneous cyclization and dehydration of 3-butene-1,2-diol (1,2-diol) or 2-butene-1,4-diol (1,4-diol) in a hydroxylic solvent in the presence of solid mercuric oxide. The preferred hydroxylic solvent is water. It is recommended that the 2,5-DHF thus formed be distilled as formed from the reaction mixture as the 2,5-DHF/water minimum-boiling azeotrope.
Similarly, U.S. Pat. No. 3,812,158 discloses that 2,5-DHF can be produced from 1,2-diol or 1,4-diol in a hydroxylic solvent using a soluble mercury salt such as HgSO4. Again, the preferred solvent is water. In the processes described in U.S. Pat. Nos. 3,812,158 and 4,231,941, 2,5-DHF is produced in the presence of large amounts of water as the reaction solvent.
It is known that 1,4-glycols can be dehydrated under acidic conditions to form the corresponding cyclic ether. Such processes are disclosed in U.S. Pat. No. 3,165,536, British Patent 510,949, and Singh, Klinzing, and Coull, Catalytic Dehydration of 2-Butene-1,4-diol, Ind. Eng. Chem. Res. Dev., 12(3), 1973, pages 184-189. Thus, 1,4-diol may be converted to 2,5-DHF by dehydration in either a liquid or vapor phase reaction in the presence of a heterogeneous or a homogeneous acidic catalyst, e.g, sulfuric acid, aluminum oxide, and montmorillonite. Although water-free 1,4-diol can be used as the starting material, the ring closure of the 1,4-diol produces one mole of water for each mole of 2,5-DHF formed. In fact, U.S. Pat. No. 3,165,536 and British Patent 510,949 teach that the 2,5-DHF produced therein is removed continuously from the reaction mixture as the 2,5-DHF/water azeotrope.
In all of the processes cited herein above, the presence of water in the reaction mixture, either as solvent or reaction by-product, cannot be avoided. Thus, any method for the recovery of pure 2,5-DHF from such a reactor effluent must be capable of separating water from 2,5-DHF. None of the patents cited herein provide for a means of obtaining water-free 2,5-DHF in a practical and economical manner on an industrial scale.
U.S. Pat. No. 3,165,536 and British Patent 510,949 disclose that the 2,5-DHF/water azeotrope may be dehydrated with the aid of drying agents such as calcium chloride or potassium carbonate. Although an acceptable drying method in the laboratory, use of a solid drying agent is impractical and not economically practical on an industrial scale. The drying agent is expensive, has low capacity for water, and is difficult to reuse, as the water bound therein to the solid drying agent can be removed only by high temperature treatment.
U.S. Pat. No. 3,165,536 and British Patent 510,949 further disclose that the 2,5-DHF/water azeotrope may be dried by an unspecified azeotropic distillation process. It is well known in the art that 2,5-DHF and water form a minimum-boiling azeotrope with a composition of about 81 mole percent 2,5-DHF and a boiling point of about 63xc2x0 C. at 760 torr. Although 2,5-DHF and water are only partially miscible, the composition of the azeotrope is outside of the two-phase liquid region at convenient temperatures, e.g., between the freezing point of water and the boiling point of 2,5-DHF. Thus, 2,5-DHF/water solutions cannot be dehydrated to low levels of water by conventional azeotropic distillation techniques employing a dual-column/decanter distillation scheme. An example of such an azeotropic distillation is the dehydration of n-butanol as taught by Wankat in Equilibrium Staged Separations, Elsevier Science Publishing Co., 1988, pages 309-312.
Extractive distillation is a method of separating close boiling or azeotrope-forming compounds from each other by carrying out the distillation in a multi-stage rectification column in the presence of an added liquid or mixture of liquids, wherein the liquid(s) have a boiling point higher than the compounds to be separated. The extractive agent is introduced near the top of the column and flows downward to the stillpot or reboiler. Its presence at all points in the liquid phase in the rectification column alters the liquid-phase behavior, and thus the relative volatility of the azeotrope-forming compounds in a manner such that a greater degree of separation than the composition of the azeotrope is possible. The extractive distillation agent should boil higher, usually substantially higher, than any of the azeotrope-forming compounds to be separated. This ensures that the extractive distillation agent remains largely in the liquid phase throughout the column. Usually the extractive agent is introduced a few stages from the top of the column to ensure that the extractive agent is not carried into the distillate with the more volatile component. In order to make subsequent recovery of the extractive agent from the less volatile component of the azeotropic mixture easier and to avoid carry-over of the extractive agent into the distillate, usually it is desirable that the extractive agent does not form azeotropes, either maximum or minimum-boiling, with any of the compounds to be separated. These criteria usually require that the extractive agent boil about 20xc2x0 C. or more above the boiling point of the lowest boiling azeotrope.
At the bottom of a continuous column, the less volatile component of the azeotropic mixture and the extractive agent are continuously removed from the column. The usual methods of separation of these two components are by a second distillation step, cooling and phase separation, or solvent extraction. For extractive distillation to be an attractive method for separating 2,5-DHF and water, an extractive distillation solvent must (1) create in their presence a large relative volatility between 2,5-DHF and water, and (2) be easy to recover from 2,5-DHF or water.
The usual method of evaluating the effectiveness of extractive distillation agents is to measure the change in relative volatility of the components to be separated in the presence of the candidate agent. Table 1 shows the degree of separation or purity obtainable by theoretical equilibrium stages at several relative volatilities. Table 1 shows that a relative volatility of at least 1.2 is required to get an effective separation by distillation. As the relative volatility is decreased below about 1.2, the number of stages required to effect the separation becomes exponentially larger and the resultant column cost is usually uneconomical. In Table I, Purity is the degree of separation or purity of both products (mole fraction).
I have developed a process or method of extractive distillation wherein the relative volatility between 2,5-DHF and water is enhanced to improve the rectification efficiency over that of a single-feed distillation column or reactive distillation column. The present invention provides a extractive distillation process for recovering 2,5-DHF from a mixture comprising 2,5-DHF and water which comprises the steps of:
(1) feeding a mixture comprising 2,5-DHF and water to the mid- or lower section of a distillation column;
(2) feeding an extractive distillation solvent to the upper section of the distillation column;
(3) removing from the upper section or top of the distillation column a vapor comprising 2,5-DHF; and
(4) removing from the lower section or bottom of the distillation column a liquid comprising water and the extractive distillation solvent;
wherein the extractive distillation solvent (i) is inert (non-reactive) under the distillation conditions, (ii) does not form an azeotrope with 2,5-DHF and/or water, (iii) is miscible with 2,5-DHF and water, (iv) has a boiling point of at least about 120xc2x0 at 760 torr. Suitable extractive distillation solvents are stable, can be separated from water or 2,5-DHF and recycled to the extractive distillation column with little decomposition, and create a large relative volatility difference between 2,5-DHF and water in their presence.